The overall goal of the Program Project is to clarify mechanisms responsible for the less effective regeneration of peripheral nerve and skeletal muscle in old compared with young animals. Project #3 will contribute to the overall goal through characterization of axonal regeneration following transection of peripheral nerves of old compared with young rats. The focus is on the coordinated, interdependent response of neurons, macrophages, and Schwann cells that is necessary for successful axonal regeneration. Project #3 will test Hypothesis C that following transection of a peripheral nerve, axonal regeneration is influenced by age-related factors intrinsic to the neuron, produced by the nerve sheath, or both. The specific hypotheses related to Hypothesis C are that after nerve injury to peripheral nerves in old compared with young rats: C-1 defective expression of neuronal genes results in impaired axonal regeneration; C-2 degeneration and clearance of myelin are delayed because of an impaired ability to attract macrophages; C-3 impaired regeneration in old rats is due in part to the failure of Schwann cells to support axonal regeneration. Two experimental models will be employed. First, sciatic nerve transection with epineurial repair in young (Y-Y-Y nerve grafts) and old (O-O-O nerve grafts) rats. Comparison will be made of the age-related response to peripheral nerve injury of neurons and Schwann cells. Alterations in gene expression will be determined by Northern blot, in situ hybridization, Western blot and light and electron microscopic immunocytochemistry. Second, cross-age nerve grafts with nerve segments from young rats grafted into the transected nerve of old rats (O-Y-O nerve grafts) and nerve segments from old rats grafted into nerves of young rats (Y-O-Y nerve grafts). The cross-age nerve graft will be used to separate the role of intrinsic neuronal and Schwann cell-produced factors on the slowing of regeneration with aging. The production of macrophage chemoattractant factors by distal nerve will be measured both in vivo and in vitro. The results of these studies will provide insights into the normal repair mechanisms of peripheral nerve and will help define the factors involved in the progressive degeneration and defective regeneration characteristic of the peripheral nervous system in aging. Ultimately, this understanding will allow the development of targeted therapies to improve nerve recovery from injury in the elderly.